1. Field of the Invention
The invention relates to a process and a guidance system for ensuring reliable guidance of a vehicle along a route to a predetermined destination point, in which the current location of the vehicle is ascertained and a route is determined in the form of consecutive path points comprising at least the geographic coordinates. The route is displayed to the vehicle driver in the form of driving instructions. Any departure from the preestablished route is signalled as soon as a preestablished decision criterion is met.
2. Description of the Prior Art
Vehicle guidance systems with visible and/or audible output devices for driving instructions to guide a vehicle along an advantageous determined route to a preestablished destination point are known. For drivers unfamiliar with an area, for example, these systems display the current location and the route, as well as all impending changes in direction, in the form of driving instructions based on digitalized road maps stored in the vehicle or externally.
DE 4039887 A1 discloses a process wherein the current location is ascertained by a locating system installed in the vehicle. A route leading from the current location to a preestablished destination point is determined by at least one external computer of an off-board navigational system. This route is transmitted to the vehicle in the form of a sequence of road segments to be traversed (a so-called guide vector chain), together with a road map of the immediate surroundings, and is displayed on a display device. The external computers are installed in spatially distributed beacons and can also be connected to a traffic computer. The guide vector chain comprises the geographic coordinates of the road segments to be traversed, and thus makes it possible to compare these geographic coordinates with the current location of the vehicle. Based on the results of this comparison, it is possible to display the current location of the vehicle, together with driving instructions, to the driver on the display device, and thus to successively guide the driver to the preestablished destination. Thus, in this process, guidance is always provided starting from the current location of the vehicle, regardless of whether or not the vehicle has deviated from the preestablished route of travel.
Although this process is suitable for guiding vehicles with the help of an off-board navigation system, in particular, it does not detect deviations from the preestablished route and thus does not communicate such deviations to the driver. In this process, only when a beacon is passed does the computer select, based on the current location of the vehicle (or the beacon), at least one guide vector chain leading to the preestablished destination from a guide vector chain collective. The selected guide vector chain is then transmitted to the vehicle and displayed to the driver directly or in the form of driving instructions. This is how "error correction" is undertaken in the event of deviations from the original route.
DE 36 45 100 C2 discloses a navigation system for motor vehicles in which a map memory device must be carried in the vehicle and the route is established as a sequence of so-called "highlighted" points. This system has sensors in the vehicle that ascertain the direction of travel as well as the path that has been travelled by the vehicle. The system is therefore able to reproduce the geometric shape of the traversed path, so that, starting from an initial point, the current location of the vehicle is ascertained. Due to the limited accuracy of the data collection, there is inevitably an ongoing summation of error variables. To prevent these errors from assuming uncontrollable orders of magnitude in the current location, this navigation system, when a new "highlighted" path point is reached, replaces the data of the computed location with the actual location data of the path point reached. The fact that a new path point has indeed been reached is recognized because the length of the path between each two consecutive "highlighted" path points (i.e., the road distance) is recorded in the map memory device as a target value. The path actually travelled by the vehicle between two such path points is continually subtracted from this target value. When the difference obtained in this manner approaches zero, the "added up" location is supposed to correspond to the location of the aimed-at path point preestablished by the map memory device. If, instead, an impermissibly large difference results, the system issues a warning that the predetermined route has been deviated from. Such a warning is also issued as soon as the "added up" location of the vehicle, during a trip between the two adjacent "highlighted" path points, leaves a defined error zone around these path points. The shape of this error zone is substantially rectangular. Its width (at a right angle to the road) is half as large as the linear distance between the two path points. In the event of unplanned detours, this system, due to the relatively broad error zone between the consecutive path points, may not recognize until very late (for example, not until the next path point should have been reached) deviations that have occurred from the preestablished route.